Conventional air-brakes used on trucks and the like typically comprise six basic elements; namely, a brake chamber assembly, a push-rod, a slack-adjuster lever, an S-Cam, brake shoes, and a brake drum.
A typical air brake assembly, has a pair of brake shoes mounted within a brake drum. The brake shoes are each pivotally mounted at one end (their lowermost ends in the examples given herein). An S-Cam, which is a rotatable cam shaft having an S-shaped pair of cam surfaces at one end is mounted with the cam surfaces between the brake shoes at their other ends (their uppermost ends in the examples given herein). Cam followers on the brake shoes bear against the cam surfaces on the S-Cam. When the S-Cam is rotated the cam followers follow the contour of the cam surfaces. When the S-Cam is rotated in a sense which tends to force the cam followers apart as they follow the contours of the cam surfaces, the brake shoes pivot outwardly to bring brake linings on the outer surfaces of the brake shoes into forceful contact with the interior surface of the brake drum.
The S-Cam in a typical brake assembly is rotated by applying force to the end of the slack adjuster lever which is a rigid arm extending perpendicularly from the S-Cam at its end away from the cam surfaces. Force is applied to the slack adjuster lever by the push-rod. One end of the push-rod is pivotally connected to the end of the slack adjuster lever and the other end of the push-rod is connected to a diaphragm inside the brake chamber assembly.
To apply the brakes, pressurized air is introduced into the brake chamber assembly. The pressurized air acts on the diaphragm which, in turn, pushes on the push-rod. The push-rod acts on the end of the slack adjuster lever to rotate the S-Cam. Rotation of the S-Cam causes the brake linings to frictionally engage the inside surface of the brake drum.
Air-brakes comprising such a S-Cam arrangement (hereinafter "S-Cam air brakes") can fail if the brake linings or drum become excessively worn, or if the brakes are not properly adjusted. To properly understand this invention it is necessary to keep separate the concepts of brake wear and brake adjustment.
Brake wear refers to the loss of material from the brake linings and brake drum by frictional abrasion of the brake linings and brake drum during repeated application of the brakes. If brake linings are not regularly replaced, they may abrade to the point where the metal surface of the brake shoe (to which the brake linings are mounted) is forced into engagement with the interior surface of the brake drum when the brakes are applied. When the brake linings are worn to this degree braking is effectively lost. It is therefore important to monitor the state of wear of the brake linings and to replace them before they become too thin. The thickness of the brake linings at a given time is an indication of the amount of brake wear. Occasionally, the brake drum must be replaced as the interior surface of the brake drum (the surface which the brake linings frictionally engage when the brakes are applied) eventually becomes worn.
As the brake linings and brake drum are abraded by repeated application of the brakes, the brake linings must be moved further apart before they engage the inner surface of the brake drum. Therefore, as the brakes wear, the S-Cam must be rotated through an increasingly large angle to force the cam followers on the brake shoes far enough apart to force the brake linings against the brake drum. The required angular rotation of the S-Cam becomes greater as brake wear progresses. This process accelerates as the brake linings become worn. As the brake linings wear down the rate of wear increases because the thin, worn down linings cannot absorb the heat generated by friction between the brake linings and brake drum when the brakes are applied as well as new linings can.
Wear of the brake linings and brake drums is normal and expected and does not cause problems if the brakes are inspected and adjusted frequently. Unfortunately, all too often, truck owners do not have the brakes on their trucks inspected either carefully enough or with sufficient regularity to avoid occasional brake failures.
As the brake linings wear, two situations can cause brake failure apart from the complete abrasion away of the brake linings. The less common occurrence is that the brake linings and brake drum wear to such a point that the cam surfaces on the S-Cam cannot force the brake shoes far enough apart to engage the remaining brake lining with the brake drum no matter how far the S-Cam is rotated. An attempt to do so creates so-called "cam-over". Cam-over occurs when the S-Cam is rotated past the point where the cam followers on the brake shoes are most widely separated. At this point the cam followers either break free of the cam surfaces on the S-Cam or become jammed against the ends of the cam surfaces as the cam surfaces are forced from a normally generally vertical orientation into a horizontally disposed orientation between the cam followers. Cam-over is a result of excessive wear of the brake lining or brake drum.
A more common occurrence is that the brakes fall out of adjustment. As described above, as the brake linings and brake drum wear, the S-Cam must be rotated through an increasingly large angle to bring the brake linings into contact with the inner surface of the brake drum. As this process continues, a point is reached where the brake linings cannot be forced against the brake drum with sufficient force to stop the vehicle even when the push-rod travels through its full useful range of motion. This is because the push-rod has a very limited range of motion. Typical brake chamber assemblies will drive the push-rod over a distance which is typically less than a few inches. The limited travel of the push-rod necessitates brake adjustment.
Because the brake chamber assembly can only move the push-rod a limited distance, an operator must continually check the brake adjustment to guard against brake failure. Brake manufacturers typically publish push-rod travel distance specifications so that operators may determine, by measuring the length of the push-rod travel when the brakes are engaged, when the brakes are out of adjustment. If the push-rod is approaching its travel limits when the brakes are engaged then the brakes must be adjusted. If the operator fails to adjust the brakes when the push-rod is approaching its travel limits then the push-rod may be unable to rotate the S-Cam far enough, within the push-rod travel limits, to engage the brake linings with the brake drum with enough force to stop the vehicle.
Adjusting the brakes involves rotating the S-Cam with respect to the slack adjuster lever with the brakes not applied to reduce the clearance between the brake linings and the brake drum (i.e. to force the brake shoes farther apart). This is accomplished by means of a slack adjusting mechanism which is typically built into the slack adjuster lever. When the brakes are properly adjusted, the push-rod, within its limited range of motion, can rotate the S-Cam enough to forcefully engage the brake linings with the brake drum when the brakes are applied and yet the brake linings are not in contact with the brake drum when the brakes are not applied.
A difficulty with brake adjustment is that there is no indication to an operator attempting to adjust the brakes whether the slack adjuster assembly is being adjusted in the correct direction, or whether the slack adjuster assembly is being in fact adjusted so as to move the brake linings away from the brake drum. Further, there is no indication to an operator whether a slack adjuster assembly is working even if the slack adjuster assembly is being adjusted in the correct direction.
Brake wear and brake adjustment are interrelated, each a component of the S-Cam air brake maintenance cycle. The cycle is thus:
(1) When new brake linings are installed, the S-Cam is returned to its unrotated position with respect to the slack adjuster lever by means of the slack adjuster assembly. Because the brake linings are new, and thus are relatively thick, the S-Cam only has to rotate a very small distance in order to engage the fresh brake lining surface with the interior of the brake drum. PA1 (2) As the brakes are repeatedly applied and the brake linings (and drum) wear down, the push-rod has to travel further on each application of the brakes to push the brake shoes sufficiently far apart (by rotating the S-Cam) to forcefully engage the brake linings with the brake drum. Eventually, after a first series of brake applications, the brake linings wear sufficiently so that, the push-rod is near its travel limit when the brakes are fully applied. PA1 (3) With the brakes off (i.e. with the push-rod at the beginning of its stroke), the brakes are adjusted by rotating the S-Cam relative to the slack adjuster lever by means of the slack adjuster assembly to force the brake shoes apart. This brings the brake linings into proximity with the interior surface of the brake drum within the manufacturer's specified clearance limits. PA1 (4) Steps 2 and 3 of the cycle then repeat with the next series of brake applications until the push-rod once again approaches its travel limit when the brakes are applied. After repeated cycles, the brake linings must be replaced before they are either completely worn out or the S-Cam approaches cam-over.
Conventionally, to check brake lining wear, a mechanic must inspect the brake lining through an inspection hole in the brake backing plate.
To check brake adjustment the push-rod travel must be measured and compared against the manufacturers specifications. Such inspections must be carried out for each brake assembly. A standard tractor-trailer might have ten or more such assemblies. The operator is expected to frequently inspect all of the brake assembles, day and night, in any weather. To properly conduct the inspections, the operator must usually crawl underneath the truck. It is not surprising that quite often only a cursory inspection is conducted.
The problem of warning vehicle operator when the brakes are almost out of adjustment is recognized in the prior art. Prior art devices simplify the operator's task of measuring push-rod travel but nonetheless rely on the measurement or monitoring of the physical travel of the push-rod. Typically the push-rod has markings placed either on or near it so that an operator can quickly check the displacement of the push-rod with respect to the markings to determine brake adjustment. Prior art devices have also included switches which are tripped by the push-rod when the push-rod approaches its travel limits so as to warn the operator of impending brake failure.
U.S. Pat. No. 5,044,302, Goldfein et al., dated 3 Sep., 1991, discloses measuring push-rod travel as a means of measuring brake lining wear. U.S. Pat. No. 4,776,438, Schandelmeier, dated 11 Oct., 1988, and U.S. Pat. No. 4,757,300, Sebalos, dated 12 Jul., 1988, disclose measuring push-rod travel as a means of measuring brake adjustment. These examples of prior art do not mention or imply that brake wear and adjustment may be monitored by measuring the rotation of the S-Cam in accordance with the present invention.
Various other methods and devices, apart from the use of an inspection hole in the brake backing plate, are found in the prior art for allowing an operator to determine brake wear. One such device is found in U.S. Pat. No. 3,356,188, Goldman et al., which issued 5 Dec., 1967. Goldman et al. teaches using the rotational position of the S-Cam relative to the slack adjuster lever as an indication of the degree of wear of brake linings in S-Cam air brakes.
Goldman et al. provides a dial and an indicating plate. The dial attaches to the S-Cam shaft and the indicating plate attaches to the slack adjuster lever. When new brake linings are installed and the brakes are properly adjusted a central lip on the dial corresponds with a tang on the indicating plate. As the brakes wear and the slack adjuster is adjusted the dial rotates with respect to the indicating plate. When the brake linings are completely worn and the brakes are properly adjusted, the tang corresponds with one of two lips on the dial.
As described above, monitoring brake wear merely means that the remaining thickness of the brake linings is being monitored. Thus in the Goldman et al. device, the relative movement of the dial over the indexing plate on the slack adjuster lever is an indication of how much the S-Cam has been rotated in adjusting the slack adjuster from its fully backed off position (the position equated with new unworn brake linings) to the position where the brake shoes have the proper clearance from the brake drum so that the brake shoes will properly engage the brake drums when the brakes are actuated. The amount the S-Cam shaft has been rotated relative to the slack adjuster lever to achieve this position is indicated by the position of the dial over the indexed plate.
The Goldman et al. device cannot be used to determine brake adjustment. As explained above, brake adjustment depends upon the physical travel limits of the push-rod and, in particular whether, within the short push-rod stroke, the brake shoes are forced outwards into forceful frictional engagement with the brake drum so as to provide effective braking.
The Goldman et al. device will not properly indicate the two problems caused by a limited push-rod stroke. That is, first, if the slack adjuster assembly has not been recently adjusted then there is no way to tell whether, within the short push-rod stroke, the S-Cam rotates far enough to force what is left of the brake linings against the brake drum. If the push-rod reaches the end of its stroke before the brake linings are brought into frictional engagement with the brake drum, the brakes are out of adjustment and will not operate properly. Goldman et al. does not teach monitoring brake adjustment, merely brake wear through cumulative S-Cam rotation. Second, if the S-Cam has been rotated too far by repeated adjustment of the slack adjuster assembly, further rotation of the S-Cam caused by actuating the push-rod may cause cam-over. The Goldman et al. does not teach monitoring for impending cam-over because it monitors S-Cam rotation only relative to the slack adjuster lever and not relative to a fixed reference such as the vehicle frame. The Goldman et al. device merely provides indication of the degree of brake wear. Consequently, brakes incorporating the Goldman et al. device may be out of adjustment or approaching cam-over and an operator looking at the Goldman et al. device would not be properly warned of impending brake failure.
U.S. Pat. No. 4,937,554, Herman, dated 26 Jun., 1990 discloses an electronic brake monitoring system which displays the status of the brakes in the vehicle cab. A computer monitors fluid pressure in the vehicle brakes, and more particularly, in the brakes pressurized fluid chamber. Herman does not teach or suggest monitoring rotation of the S-Cam.